1. Field of the Invention
The present invention relates generally to thermal barrier coatings, and more specifically to a test rig for testing various thermal barrier coatings under operating conditions.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Existing known techniques for testing materials to be used in a gas turbine engine are very expensive or do not properly expose the testing material to actual engine operating conditions. One known method is to use an actual gas turbine engine and place the material to be tested on a part in the engine while the engine is operating. This method requires an operating gas turbine engine that is very expensive to operate. An engine test can test a material or a coating for: spallation due to high thermal gradients; erosion due to high velocity flow; corrosion degradation due to trace elements in fuel at operating temperatures and pressures; and, includes the ability to apply axial loading in addition to thermal loading to the test specimen. However, the engine test method is very expensive to operate (about $6,000 per hour to operate), the test conditions are limited to current technologies (pressures, temperatures, stresses) used in the specific testing gas turbine engine, availability of engine hardware, engine test facility, and large staffing requirements, and limited hot time accumulated (generally less than 300 hours).
A less costly method of testing that does not require an operating gas turbine engine is a burner rig. Existing rigs for testing turbine coating/material combinations use a hot flame impingement onto a material/coating specimen to ascertain material/coating durability under hot conditions. While these burner tests are more easily accomplished than full engine tests, are typically of low cost, and are sometimes satisfactory as a screening method, they fail to duplicate many of the parameters leading to material/coating failures observed in actual component designs. Of the conditions described above with respect to the engine test method, a burner rig can provide for a low cost method of testing materials, the burner rig does not allow for the testing for: coating spallation due to high thermal gradients; for erosion to high velocity flow; or for the ability to apply axial loading in addition to thermal loading to the test material.
Realistic engine gas path conditions include high thermal gradients in the test specimen, thermal and mechanical fatigue loading, and erosion due to high velocity gas flow. In real engines, the coating/material components are subjected to cyclical mechanical loading that can affect metal and coating durability and coating adhesion. In addition, the hot gas often contains trace contaminants that can cause corrosion of the metal/coating systems. High velocity gas flows can erode the gas path materials which also reduce their durability. Burner rigs are limited in that no mechanical loading can be applied to the specimen, and that the flow is not at high velocity so that TMF and erosion mechanisms are not duplicated in the test system.
Other complex systems are being developed for advanced testing of gas path materials. The Westinghouse Plasma Corporation's facility in Waltz Mills, Pa. uses a plasma torch to heat material specimens to high thermal loading and also includes mechanical loading capability to simulate TMF conditions. Currently the system is limited to heat flux levels less than 1.2 MBtu/hr/ft.sup.2. The system is also not able to support investigation of erosion failure mechanisms since there is no high velocity flow. Moreover, the ability to accurately measure temperature on the front and backsides of the specimen (to determine thermal gradient) is questionable.
A third system is under development by NASA as part of the Ultra Efficient Engine Technology (UEET) program. This system uses a laser generated heat flux to heat the specimen to high thermal gradients. The current system can achieve approximately 1 MBtu/hr/ft.sup.2. It is unknown if mechanical loading can be applied to the specimen, however, the system is limited in its ability to duplicate erosion failure mechanisms. Further, the system is not pressurized, but does have cooling through the middle of the specimen.
The degradation process that require characterization include coating erosion, spallation, thermal mechanical fatigue, low cycle fatigue, hold-time effects, as well as the interaction of these failure mechanisms. With extremely high cost of developing a new engine concept, especially when operating conditions will exceed all current experience, low cost test rigs are the prudent way to screen new concepts and materials prior to committing to actual engine hardware and full engine testing.
U.S. Pat. No. 7,174,797 issued to Brostmeyer et al on Feb. 13, 2007 and entitled HIGH TEMPERATURE AND PRESSURE TESTING FACILITY discloses a test facility for testing materials under high temperature, pressure, and mechanical loads. The facility provides a physically scaled system that simulates conditions in hot sections of gas turbine engines. A test article is coated with a test material and exposed to a hot combusting flow in a test section housing. The article may be a pipe or conduit member oriented at any direction to the flow. A second cooler flow of fluid is channeled through the test article to create a sharp temperature gradient in the test article and through the test material. A liquid-cooled sleeve is disposed about the test article to create an annular channel of combusting flow over the test article. The downstream end of the second cooler flow is connected to the upstream end of the main hot flow at the combustion chamber. The Brostmeyer et al test rig does not offer the capability to view the material being tested during the testing process. Also, this test rig does not offer easy access to the test material without having to disassemble the test rig.
There is a need in the prior art for a test rig that can provide a low cost way to test materials for use in gas turbine engines, as well as a test rig that can reproduce all the conditions such as high temperature, high pressure, erosion, corrosion, and thermal and mechanical loading, that occur in an operating gas turbine engine.
It is an object of the present invention to provide for an apparatus and a method that can test materials at a very low cost.
It is another object of the present invention to provide for an apparatus and a method to test materials under the extreme conditions operating in a gas turbine engine.
It is another object of the present invention to provide for an apparatus and a method that can test materials at temperatures above the maximum temperature permitted by today's material limitations.
It is another object of the present invention to provide for an apparatus and a method that can test materials under axial and thermal loadings.
It is another object of the present invention to provide for an apparatus and a method to test materials under the extreme conditions operating in a gas turbine engine in which the material being tested can be visually observed during the testing.
It is another object of the present invention to provide for an apparatus and a method to test materials under the extreme conditions operating in a gas turbine engine in which the exhaust gas flow from the test rig has been cooled enough to prevent thermal damage to valves and conduits downstream from the test rig.
It is another object of the present invention to provide for an apparatus and a method to test materials under the extreme conditions operating in a gas turbine engine in which the hot gas flow passing over the material to be tested is clear so that the material being tested can be seen.